1. Field of the Invention
This invention relates to a method of applying controlled cooling on hot steel plates, and more particularly to a method of applying controlled cooling on hot steel plates without impairing the shape thereof.
2. Description of the Prior Art
With the reduction of alloying elements, efficient utilization of heat, and development of new steels in view, many studies have been made recently on what are generally called thermal-refining cooling processes for plate production in which heating with controlled temperature and time, controlled rolling, and forced cooling immediately after rolling are combined.
The sequence of controls exercised in the heating and cooling processes are intended for achieving regulation of the transformation characteristic of steel plates and enhancement of their mechanical properties. With the metallurgical mechanisms almost fully clarified, controlled heating and rolling technologies have been widely adopted during the past 10 years as in-line production processes, principally in the manufacture of high-strength line pipe steels for low-temperature and cryogenic services. For forced cooling technology, on the other hand, temperature and shape controls have not yet reached a level high enough to permit in-line incorporation and stable operation, though adequate light has been thrown upon the metallurgical mechanisms thereof.
Forced cooling of hot steel plate is done by injecting cooling water onto both surfaces of the plate through a group of nozzles disposed widthwise over and below the plate. If the injection rate is the same across the entire plate width, significant temperature difference occurs between the edges and middle portion of the plate because the former gets cooled faster than the latter. The result is the impairment of plate shape due to waviness in edges and the middle, camber and other configurational irregularities.
The U.S. Pat. No. 4,440,584 discloses a method and apparatus for cooling steel plates proposed as a solution for the problem of the kind just described. According to this technology, steel plate is cooled by cutting off the supply of cooling water to the upper surface of the edge portion of the plate being cooled so that uniform widthwise temperature distribution is achieved on completion of cooling to prevent the deformation of the plate after cooling.
However, the inventors have found that the deformation of the plate cannot be fully prevented even if the supply of cooling water to the upper surface of the edge portion is cut off.
The inventors have also found that unless cooling is effected with the time at which austenite transformation at Ar.sub.3 begins in the edge and middle portions in mind, great residual stress would result from the abrupt changes in the coefficient of linear expansion and yield stress that occurs with that transformation. When the plate is cooled down to ambient temperature, the residual stress will cause great enough deformation to impair the shape of the product plate beyond the tolerable limit.